This invention relates to ac generators for vehicles, and more particularly to an ac generator of the kind which has an inexpensive stator frame of steel plate and is thus suitable for use in automobiles.
Economization of industrial resources is presently strongly called for, and an attempt is now being made to employ inexpensive and readily available steel plates for the fabrication of stator frames of rotary electric machines of relatively small size. In spite of such tendency, however, die castings of aluminum are presently widely used for the greater part of conventional stator frames of ac generators for automobiles for various reasons as enumerated below. In the first place, a stator frame structure has not had sufficient mechanical strength to withstand vibration of large amplitude when a steel plate is used to form the stator frame of such rotary electric machine. Especially, a stator unit constituted by integral connection of a cylindrical casing of steel plate, a stator core, flanges and end covers of steel plate has been defective in that such stator unit tends to resonate with engine or vehicle body vibration resulting in breakage of the flanges, loosening of the stator core fixed to the cylindrical casing, etc. Secondly, the bolts used for fixing the cylindrical casing to the end covers have tended to become loose, and an attempt to prevent this undesirable loosening has resulted in an uneconomical increase in the magnetic losses of the magnetic circuit. Thirdly, although a method of fixing the stator core to the cylindrical casing by welding the flanges to the cylindrical casing, forcing the stator core having the stator widning thereon into the cylindrical casing, and then bolting the stator core to the cylindrical casing has been proposed, this method has been defective in that the stator unit thus obtained is mechanically weak against vibration of large amplitude, and the bolts fixing the stator core to the cylindrical casing tend to be loosened. In an effort to obviate the above defect, another method has been proposed in which the stator core is fixed to the cylindrical casing by welding. However, due to the fact that the radial thickness of the stator core used in the ac generator of this kind is of the order of 5 to 6 mm and is thus very small compared with the thickness of 10 to 20 mm of the stator core in small-sized induction motors, it has been impossible to prevent the widning and insulator portions of the ac generator from being extremely deteriorated during welding.